Historically, many of the first exoplanets to be discovered were found via the radial velocity method.
Using super-precise and ultra-well calibrated instruments, astronomers searched for the tiny wobble in the star's speed caused by the planet's orbit. Other methods at exoplanet astronomers' disposals include detecting gravitational lensing due to a planet called the microlensing method , searching for the wobble in the star's position on the sky called the astrometric method , and separating the light of the star from the planet and actually taking images called the direct imaging method.
 High-precision photometry of WASP b transits
This tutorial focuses on the transit method , where we search for the period dimming of light as an exoplanet passes in front of its host star and casts its shadow on our telescopes. An animation below illustrates what happens when a planet transits:.
In this animation, the green line tracing below the planet and star is called the "light curve. The dip in light that happens when the planet passes in front of the star is called the "transit.
The animation below shows a planet transiting three times successively before the transits are detected by Kepler :. The length of time between each transit is the planet's "orbital period", or the length of a year on that particular planet. Not all planets have years as long as a year on the Earth!
Some planets discovered by Kepler orbit around their stars so quickly that their years only last about four hours! Perhaps the most important aspect of transiting planets is that you can measure the planet's size. This measurement is extremely difficult to make if the planet does not happen to transit. The amount of light a planet blocks in transit depends on the size of the planet. A large planet blocks more light than a small planet, so the large planet will have a deeper transit.
This is illustrated in the animation below:. Transits can only be observed with properly equipped telescopes.
Transits of Mercury
By no means, do not try to observe these event without being trained! You may damage your eyes. The much smaller disk of the planet starts entering the bright solar disk. This is a partial solar eclipse.
The apparently small planetary disk seems to hover in front of the Sun, completely detached from the border of the limb. Maximum eclipse Gives the object name Venus or Mercury , and time and phase of maximum eclipse, i. With the latter, the planetary disk is apparently floating as a small dot across the disk. Hard copies may be made for personal use only. No electronic copy may be located elsewhere for public access.
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